Conventional means for delivering biologically-active agents, including, but not limited to, pharmaceutical and therapeutic agents, to animals are often severely limited by chemical barriers and physical barriers imposed by the body. Oral delivery of many biologically-active agents would be the route of choice if not for chemical and physico-chemical barriers such as the extreme and varying pH in the gastro-intestinal (GI) tract, exposure to powerful digestive enzymes, and the impermeability of gastro-intestinal membranes to the active agent. Among the numerous agents which are not typically suitable for oral administration are biologically active peptides such as calcitonin and insulin. Examples of other compounds which are affected by these physico-chemical barriers are polysaccharides and particularly mucopolysaccharides, including, but not limited to, heparin; heparinoids; antibiotics; and other organic substances. These agents are rapidly destroyed in the gastro-intestinal tract by acid hydrolysis, enzymes, or the like.
Prior methods for orally administering vulnerable pharmacological agents have relied on the co-administration of adjuvants (e.g., resorcinols and non-ionic surfactants such as polyoxyethylene oleyl ether and n-hexadecyl polyethylene ether) to increase artificially the permeability of the intestinal walls; and on the co-administration of enzymatic inhibitors (e.g., pancreatic trypsin inhibitor, diisopropylfluorophosphate (DFF) and trasylol) to inhibit enzymatic degradation. Liposomes have also been described as drug delivery systems for insulin and heparin. See, for instance, U.S. Pat. No. 4,239,754; Patel et al. (1976) FEBS Letters Vol. 62, page 60; and Hashimoto et al. (1979) Endocrinol. Japan, Vol. 26, page 337. However, broad spectrum use of the aforementioned drug delivery systems is precluded for reasons including: (1) the need to use toxic amounts of adjuvants or inhibitors; (2) the lack of suitable low MW cargoes; (3) the poor stability and inadequate shelf life of the systems; (4) the difficulties in manufacturing the systems; (5) the failure of the systems to protect the active ingredient; and (6) the failure of the systems to promote absorption of the active agent.
More recently, microspheres of artificial polymers or proteinoids of mixed amino acids have been described for delivery of pharmaceuticals. For example, U.S. Pat. No. 4,925,673 describes drug containing microsphere constructs as well as methods for their preparation and use. These proteinoid microspheres are useful for delivery of a number of active agents.
There is still a need in the art for simple and inexpensive delivery systems which are easily prepared and which can deliver a broad range of biologically-active agents.